Papers by Author: K. Rumpf

Abstract: Industrial available FeSi, FeCo and FeNi alloys with an initial grain size of 20-50 m were subjected to Severe Plastic Deformation (SPD) up to strain levels where a saturation of the microstructural refinement is observed. For both SPD conditions, ambient temperature (293 K) and liquid nitrogen temperature (77 K), the microstructure of the severely deformed state is analysed by Back Scattered Electrons (BSE) micrographs captured in a SEM. Additionally, samples that were deformed at 77 K are examined in a Transmission Electron Microscope (TEM). The magnetic properties were characterised by means of SQUID-magnetography providing information about the magnetization behaviour of the material in the as-processed state. Depending on the SPD conditions the mean microstructural sizes in the steady state are about 100-150 nm and 30-80 nm at 293 K and 77 K, respectively. The small microstructural sizes influence significantly the magnetic properties of these ferritic alloys. The initial soft-magnetic behaviour of the coarse grained state shifts towards a hard-magnetic with decreasing crystallite size. For crystallite sizes smaller than ~80 nm the magnetic properties become again more soft-magnetic. The experiments show that very low coercitivity can be obtained by SPD if the grain size is smaller than ~50 nm.

Abstract: A commercial FeCo ferritic steel with an initial grain size of 10 μm was subjected to Severe Plastic Deformation in a temperature range between 293 K (0.16Tm, Tm: melting temperature in K) and 723 K (0.4Tm) up to strain levels where a saturation of the microstructural refinement is observed. The microstructure of the severely deformed state is analyzed by Back Scattered Electrons micrographs captured in a SEM. The magnetic properties were characterized by means of SQUID-magnetometer providing information about the magnetization behavior of the material in the as processed state. Depending on the deformation temperature mean microstructural sizes in the steady state of 50 nm and 270 nm were observed after SPD at 293 K and 723 K, respectively. These small microstructural sizes influences significantly the magnetic properties of the material: it shifts the behavior from soft-magnetic in the initial coarse grained state towards a hard-magnetic with decreasing size of the crystallites. For sizes of the crystallites smaller than about 100 nm the magnetic properties become again more soft-magnetic.